[Koha] Koha not allowing renewals, help needed

[Chris Brown]

Gentle Reader,

I am having a problem with a Koha system I've set up for training.
For all books and all patrons the "Renew" column of the checkout table
shows "Not renewable (0 of 0 renewals remaining)"

I have only one circulation rule; it has the number of renewals set to 3.
The "No renewal before" field is blank.

Some other potentially relevant sysprefs are as follows:

OverduesBlockRenewing  is set to "allow renewing"
RestrictionsBlockRenewing is set to "Allow"

A training system for another library that is hosted on the same server, and
which is identically configured as far as I can see, is working fine.

Any suggestions on what could be preventing renewals would be appreciated!

Using Koha 17.05 installed from debian packages on Debian 9

Best Regards,

Chris Brown